141.环形链表

/*
 * @lc app=leetcode.cn id=141 lang=typescript
 *
 * [141] 环形链表
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     val: number
 *     next: ListNode | null
 *     constructor(val?: number, next?: ListNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.next = (next===undefined ? null : next)
 *     }
 * }
 */

function hasCycle(head: ListNode | null): boolean {
	if (head == null) return false;
	let map: Map<ListNode, string> = new Map();

	let current: ListNode | null = head;
	while (current.next) {
		if (map.has(current)) {
			// 存在
			return true;
		}
		map.set(current, "1");
		current = current.next;
	}

    return false
}
// @lc code=end